A skirt is generally formed on the lower part of an engine piston which executes a back and forth motion, the piston being connected to a connecting rod via a piston pin which passes through the skirt. This skirt has the function of supporting the piston such that the latter does not lean due to the combustion energy acting upon It, and the skirt slides on the inner circumference of the cylinder.
As the skirt tends to suffer more thermal expansion in a perpendicular direction to the piston pin due to the combustion heat of the engine, it may for example be formed with all elliptical cross-section having the piston pin as its short axis and the perpendicular direction as its long axis.
Although the skirt has to support the piston, it must not cause the frictional resistance of the piston to increase. It is therefore desirable that the contact surface area between the skirt and the cylinder is small provided that the supporting force is still sufficient.
In Japanese Tokkai Hei 3-115762 and Tokkai Hei 3-199655 published by the Japanese Patent Office, pistons are described wherein the skirt has all elliptical cross-section which progressively approaches a circle towards the upper part of the skirt. As a result of this arrangement, the skirt has a T-shaped area on Its outer circumference to contact with the cylinder.
In order to render the piston more compact and lightweight, it is desirable that the part of the skirt under the piston pin is kept short. However, if the length of the skirt under the piston pin is short, the surface area of the skirt under the piston pin which is in contact with the cylinder is small, and the force acting on this contact surface due to the transverse load acting on the piston increases, which makes the piston unstable.